The molecular mechanisms of morphine addiction

Zingerone Alleviates Morphine Tolerance and Dependence in Mice by Reducing Oxidative Stress-Mediated NLRP3 Inflammasome Activation

Morphine (MPH) is widely used for pain management; however, long-term MPH therapy results in antinociceptive tolerance and physical dependence, limiting its clinical use. Zingerone (ZIN) is a natural phenolic compound with neuroprotective effects. We investigated the effects of single and repeated doses of ZIN on MPH-induced tolerance, dependence, and underlying biochemical mechanisms. After a dose-response experiment, tolerance was developed to MPH (10 mg/kg, i.p.) for seven days. In the single-dose study, ZIN was administered on day seven. In the repeated-dose study, ZIN was administered for seven days. Naloxone (5 mg/kg, i.p., 120 min after MPH) was injected to assess withdrawal signs on day seven. The levels of thiobarbituric acid reactive substances (TBARS), nitric oxide (NO), total thiol (TT), and glutathione peroxidase (GPx) were measured in the prefrontal cortex. The protein levels of interleukin-1 beta (IL-1β) and NLRP3-ASC-Caspase-1 axis were assessed by ELISA and Western blotting, respectively. Results showed that ZIN (100 mg/kg) had no antinociceptive activity, and subsequent experiments were performed at this dose. Repeated ZIN reversed MPH antinociceptive tolerance, whereas single ZIN did not. Single and repeated ZIN attenuated naloxone-induced jumping. In addition, repeated ZIN significantly inhibited weight loss. Repeated ZIN suppressed the MPH-induced increase in TBARS, NO, IL-1β, NLRP3, ASC, and Caspase-1. It also inhibited MPH-induced TT and GPx reduction. In contrast, single ZIN had no effect. Findings suggest that ZIN reduces MPH-induced tolerance and dependence by suppressing oxidative stress and NLRP3 inflammasome activation. This study provides a novel therapeutic approach to reduce the side effects of MPH.

Functional genomic mechanisms of opioid action and opioid use disorder: a systematic review of animal models and human studies

In the past two decades, over-prescription of opioids for pain management has driven a steep increase in opioid use disorder (OUD) and death by overdose, exerting a dramatic toll on western countries. OUD is a chronic relapsing disease associated with a lifetime struggle to control drug consumption, suggesting that opioids trigger long-lasting brain adaptations, notably through functional genomic and epigenomic mechanisms. Current understanding of these processes, however, remain scarce, and have not been previously reviewed systematically. To do so, the goal of the present work was to synthesize current knowledge on genome-wide transcriptomic and epigenetic mechanisms of opioid action, in primate and rodent species. Using a prospectively registered methodology, comprehensive literature searches were completed in PubMed, Embase, and Web of Science. Of the 2709 articles identified, 73 met our inclusion criteria and were considered for qualitative analysis. Focusing on the 5 most studied nervous system structures (nucleus accumbens, frontal cortex, whole striatum, dorsal striatum, spinal cord; 44 articles), we also conducted a quantitative analysis of differentially expressed genes, in an effort to identify a putative core transcriptional signature of opioids. Only one gene, Cdkn1a, was consistently identified in eleven studies, and globally, our results unveil surprisingly low consistency across published work, even when considering most recent single-cell approaches. Analysis of sources of variability detected significant contributions from species, brain structure, duration of opioid exposure, strain, time-point of analysis, and batch effects, but not type of opioid. To go beyond those limitations, we leveraged threshold-free methods to illustrate how genome-wide comparisons may generate new findings and hypotheses. Finally, we discuss current methodological development in the field, and their implication for future research and, ultimately, better care.

Regulation of clock and clock-controlled genes during morphine reward and reinforcement: Involvement of the period 2 circadian clock

BACKGROUND

Morphine abuse is a devastating disorder that affects millions of people worldwide, and literature evidence indicates a relationship between opioid abuse and the circadian clock.

AIM

We explored morphine reward and reinforcement using mouse models with Per2 gene modifications (knockout (KO); overexpression (OE)).

METHODS

Mice were exposed to various behavioral, electroencephalographic, pharmacological, and molecular tests to assess the effects of morphine and identify the underlying mechanisms with a focus on reward and reinforcement and the corresponding involvement of circadian and clock-controlled gene regulation.

RESULTS

Per2 deletion enhances morphine-induced analgesia, locomotor sensitization, conditioned place preference (CPP), and self-administration (SA) in mice, whereas its overexpression attenuated these effects. In addition, reduced withdrawal was observed in Per2 KO mice, whereas an augmented withdrawal response was observed in Per2 OE mice. Moreover, naloxone and SCH 23390 blocked morphine CPP in Per2 KO and wild-type (WT) mice. The rewarding (CPP) and reinforcing effects (SA) observed in morphine-conditioned and morphine self-administered Per2 KO and WT mice were accompanied by activated μ-opioid and dopamine D1 receptors and TH in the mesolimbic (VTA/NAcc) system. Furthermore, genetic modifications of Per2 in mice innately altered some clock genes in response to morphine.

CONCLUSION

These findings improve our understanding of the role of Per2 in morphine-induced psychoactive effects. Our data and those obtained in previous studies indicate that targeting Per2 may have applicability in the treatment of substance abuse.

Toxic desomorphine encephalopathy due to the use of “Krokodil”: clinical and neuroimaging features

Background: Toxic desomorphine encephalopathy (TDE) is a pathological condition that develops as a result of the intravenous use of a drug called Krokodil containing desomorphine, made in the artisanal conditions using codeine-containing drugs, organic solvents (gasoline), iodine and red phosphorus. This disease is more often observed in the CIS countries. In addition to the acute and chronic pathological conditions with the damage to various organs, the use of Krokodil is characterized by pronounced extrapyramidal manifestations in the form of dystonia, parkinsonism, postural disorders, as well as the occurrence of cognitive and affective disorders. Aims: To find the clinical and neuroimaging features of toxic desomorphine encephalopathy, as well as possible methods of its treatment. Methods: A clinical analysis of the medical documentation of 21 TDE patients (11 women and 10 men) with a history of the use of Krokodil was carried out, the patients had been under observation from 2014 to 2021. All the patients underwent a clinical physical and neurological examination, 14 of them underwent neuroimaging (brain MRI and/or MSCT). The observation of these patients revealed a number of characteristic clinical and neuroimaging features inherent in the majority of drug addicts. Results: The clinical picture of patients with TDE was dominated by movement disorders. All the patients had pronounced postural disorders and gait disturbance. Parkinsonism was observed in 20 of 21 patients. The hyperkinetic syndrome was presented in 17 patients (80.9%) and was manifested by dystonia of various localization with polymorphic manifestations. The brain MRI data taken from the Krokodil users for 3 years were characterized by symmetrical focal changes in the basal ganglia, brainstem, cerebellum and internal capsule of the thalamus in the form of an increase in the intensity of the MR signal in the T1 mode and attenuation in the T2-weighted images mode (7 of 11 cases), with the subsequent regression of these characteristics based on the results of the subsequent MRI studies. Conclusion: The study results have revealed the clinical manifestations characteristic of TDE polymorphic extrapyramidal disorders, as well as neuroimaging changes reflecting these data.

Study of the effects of controlled morphine administration for treatment of anxiety, depression and cognition impairment in morphine-addicted rats

Background:

Morphine dependency usually results in undesired outcomes such as anxiety, depression, and cognitive alterations. In this study, morphine was used to manage morphine dependence-induced anxiety, depression, and learning and memory disturbances.

Materials and Methods:

Forty rats were divided equally into five groups. Group 1 received saline for 21 days. Groups 2–5 were dependent by increasing administration of morphine (15–45 mg/kg) for 7 days. For the next 14 days, morphine was administered as the following regimen: Group 2: once daily; 45 mg/kg (positive controls), Group 3: the same dose with an increasing interval (6 h longer than the previous intervals each time), Group 4: the same dose with an irregular intervals (12, 24, 36 h intervals interchangeably), and Group 5: decreasing doses once daily (every time 2.5 mg/kg less than the former dosage). On days 22–26, elevated plus maze (EPM), open field test (OFT), forced swim test (FST), and tail suspension test (TST) were performed to investigate anxiety level and depression in animals. Between 17^th and 21^st days, Morris water maze (MWM) was used to evaluate the spatial learning and memory.

Results:

Chronic morphine administration caused depression and anxiety as observed by FST, EPM, and TST and decreased motor activity in OFT and caused impairment in learning and memory performance in MWM. Treatment with our protocol as increasing interval, irregular interval, and decreasing dosage of morphine caused marked reduction in depression, anxiety, and improved cognition performance compared with positive control group; and attenuated motor deficits in morphine-dependent rats, remarkably.

Conclusions:

Change in dosage regimens of morphine can reduce morphine-induced anxiety, depression, and cognitive impairments.

Impact of chronic morphine on delta opioid receptor-expressing neurons in the mouse hippocampus

Delta opioid (DOP) receptors participate to the control of chronic pain and emotional responses. Recent data also identified their implication in spatial memory and drug-context associations pointing to a critical role of hippocampal delta receptors. To better appreciate the impact of repeated drug exposure on their modulatory activity, we used fluorescent knock-in mice that express a functional delta receptor fused at its carboxy-terminus with the green fluorescent protein in place of the native receptor. We then tested the impact of chronic morphine treatment on the density and distribution of delta receptor-expressing cells in the hippocampus. A decrease in delta receptor-positive cell density was observed in the CA1, CA3 and dentate gyrus without alteration of the distribution across the different GABAergic populations that mainly express delta receptors. This effect partly persisted after four weeks of morphine abstinence. In addition, we observed increased DOP receptor expression at the cell surface compared to saline-treated animals. In the hippocampus, chronic morphine administration thus induces DOP receptor cellular redistribution and durably decreases delta receptor-expressing cell density. Such modifications are likely to alter hippocampal physiology, and to contribute to long-term cognitive deficits.

A Review Study on the Effect of Iranian Herbal Medicines on Opioid Withdrawal Syndrome

Addiction is a chronic and recurring disease that recurrence phenomenon is the most important challenge in treatment of this disease. Recent experiences have shown that synthetic drugs have undesirable side effects. Recent studies on medicinal plants have shown that they might be effective in treatment of different stages of addiction with lower side effects and costs. The aim of this study was to review the effects of medicinal plants in the treatment of morphine addiction in experimental animals. In this review article, by using keywords of morphine, withdrawal, and plants or herbal medicine in databases of indexing cites, desired articles were obtained since 1994. Inclusion criteria for selecting articles were the articles related to application of medicinal plants in decreasing symptoms resulting from morphine withdrawal were selected. Results of this study on experimental studies have shown that medicinal plants such as Trachyspermum copticum L and Melissa officinalis decrease the symptoms of withdrawal syndrome in a dose-dependent. Also, medicinal plants like Avena sativa, Hypericum perforatu, Passiflora incarnate, Valeriana officinalis, Satureja hortensis L, and Mentha piperita can have effects on behavior, emotions, and other problems of addicts, decreasing withdrawal symptoms. Results of this study showed that medicinal plants can be effective in controlling deprivation, decreasing dependency creation, and possibly detoxification of opioid addicts.

Changes in the expression of hippocampal proteins in rats with recrudescence of morphine addiction

The high recrudescence rate of drug addiction has received attention worldwide and its mechanisms remain to be elucidated. This study aimed to analyse the disparate protein expression in the hippocampal tissue of rats with recrudescence of morphine addiction, as well as to provide clues for the exploration of the recrudescence mechanism. Sixteen male adult Sprague-Dawley rats were divided equally into the morphine and physiological saline groups. Effective nose pokes were determined as the main index. The proteins were separated using the immobilised pH gradient two-dimensional polyacrylamide gel electrophoresis (2-DE). Disparate protein spots were analysed using the PDQuest 2-DE software. Peptide dactylograms were obtained using the matrix-assisted laser desorption/ionisation time-of-flight mass spectrometry. The effective nose poke counts of the morphine group significantly increased during addiction maturation compared with the saline group (P<0.001). The post-recrudescence nose poke counts of the morphine group significantly increased compared with those before recrudescence (P<0.001). Fifteen disparate proteins were identified according to the protein electrophoresis of the morphine and physiological saline groups, including three proteins associated with energy metabolism, two ionic channel regulatory proteins, one heat shock protein and one exogenous substance metabolic enzyme. The energy metabolism and expression of cell metabolism-related proteins decreased in the hippocampus of rats with morphine recrudescence.

Development of mechanical hypersensitivity in rats during heroin and ethanol dependence: alleviation by CRF₁ receptor antagonism

Animal models of drug dependence have described both reductions in brain reward processes and potentiation of stress-like (or anti-reward) mechanisms, including a recruitment of corticotropin-releasing factor (CRF) signaling. Accordingly, chronic exposure to opiates often leads to the development of mechanical hypersensitivity. We measured paw withdrawal thresholds (PWTs) in male Wistar rats allowed limited (short access group: ShA) or extended (long access group: LgA) access to heroin or cocaine self-administration, or in rats made dependent on ethanol via ethanol vapor exposure (ethanol-dependent group). In heroin self-administering animals, after transition to LgA conditions, thresholds were reduced to around 50% of levels observed at baseline, and were also significantly lower than thresholds measured in animals remaining on the ShA schedule. In contrast, thresholds in animals self-administering cocaine under either ShA (1 h) or LgA (6 h) conditions were unaltered. Similar to heroin LgA rats, ethanol-dependent rats also developed mechanical hypersensitivity after eight weeks of ethanol vapor exposure compared to non-dependent animals. Systemic administration of the CRF1R antagonist MPZP significantly alleviated the hypersensitivity observed in rats dependent on heroin or ethanol. The emergence of mechanical hypersensitivity with heroin and ethanol dependence may thus represent one critical drug-associated negative emotional state driving dependence on these substances. These results also suggest a recruitment of CRF-regulated nociceptive pathways associated with escalation of intake and dependence. A greater understanding of relationships between chronic drug exposure and pain-related states may provide insight into mechanisms underlying the transition to drug addiction, as well as reveal new treatment opportunities. This article is part of a Special Issue entitled 'Post-Traumatic Stress Disorder'.

Changing mechanisms of opiate tolerance and withdrawal during early development: animal models of the human experience

Human infants may be exposed to opiates through placental transfer from an opiate-using mother or through the direct administration of such drugs to relieve pain (e.g., due to illness or neonatal surgery). Infants of many species show physical dependence and tolerance to opiates. The magnitude of tolerance and the nature of withdrawal differ from those of the adult. Moreover, the mechanisms that contribute to the chronic effects of opiates are not well understood in the infant but include biological processes that are both common to and distinct from those of the adult. We review the animal research literature on the effects of chronic and acute opiate exposure in infants and identify mechanisms of withdrawal and tolerance that are similar to and different from those understood in adults. These mechanisms include opioid pharmacology, underlying neural substrates, and the involvement of other neurotransmitter systems. It appears that brain circuitry and opioid receptor types are similar but that NMDA receptor function is immature in the infant. Intracellular signaling cascades may differ but data are complicated by differences between the effects of chronic versus acute morphine treatment. Given the limited treatment options for the dependent infant patient, further study of the biological functions that are altered by chronic opiate treatment is necessary to guide evidenced-based treatment modalities.

Development of morphine-induced tolerance and withdrawal: involvement of the clock gene mPer2

The present study has been designed to assess specifically the involvement of the clock gene mPer2 in morphine-induced tolerance and withdrawal. At first, we checked the absence of initial differences in the expression of several gene transcripts involved in the development of morphine dependence in Per2(Brdm1) mutant mice and in their respective wild-type (WT) control littermates. Morphine-induced tolerance as well as precipitated withdrawal was then assessed in these mice. The Per2(Brdm1) mutant mice clearly developed less tolerance and showed attenuated withdrawal signs compared to WT. These results show that mPER2 is involved in morphine-induced tolerance and withdrawal.

Inhibition of heat shock protein 90 attenuates adenylate cyclase sensitization after chronic morphine treatment

Cellular adaptations to chronic opioid treatment result in enhanced responsiveness of adenylate cyclase and an increase in forskolin- or agonist-stimulated cAMP production. It is, however, not known whether chaperone molecules such as heat shock proteins contribute to this adenylate cyclase sensitization. Here, we report that treatment of cells with geldanamycin, an inhibitor of heat shock protein 90 (Hsp90), led to effective attenuation of morphine-induced adenylate cyclase sensitization. In SK-N-SH human neuroblastoma cells, morphine significantly increased RNA transcript and protein levels of type I adenylate cyclase, leading to sensitization. Whole-genome tiling array analysis revealed that cAMP response element-binding protein, an important mediator for cellular adaptation to morphine, associated with the proximal promoter of Hsp90AB1 not only in SK-N-SH cells but also in rat PC12 and human embryonic kidney cells. Hsp90AB1 transcript and protein levels increased significantly during morphine treatment, and co-application of geldanamycin (0.1-10 nM) effectively suppressed the increase in forskolin-activated adenylate cyclase activation by 56%. Type I adenylate cyclase, but not Hsp90AB1, underwent significant degradation during geldanamycin treatment. These results indicate that Hsp90 is a new pharmacological target for the suppression of adenylate cyclase sensitization induced by chronic morphine treatment.

Reward processing by the opioid system in the brain

The opioid system consists of three receptors, mu, delta, and kappa, which are activated by endogenous opioid peptides processed from three protein precursors, proopiomelanocortin, proenkephalin, and prodynorphin. Opioid receptors are recruited in response to natural rewarding stimuli and drugs of abuse, and both endogenous opioids and their receptors are modified as addiction develops. Mechanisms whereby aberrant activation and modifications of the opioid system contribute to drug craving and relapse remain to be clarified. This review summarizes our present knowledge on brain sites where the endogenous opioid system controls hedonic responses and is modified in response to drugs of abuse in the rodent brain. We review 1) the latest data on the anatomy of the opioid system, 2) the consequences of local intracerebral pharmacological manipulation of the opioid system on reinforced behaviors, 3) the consequences of gene knockout on reinforced behaviors and drug dependence, and 4) the consequences of chronic exposure to drugs of abuse on expression levels of opioid system genes. Future studies will establish key molecular actors of the system and neural sites where opioid peptides and receptors contribute to the onset of addictive disorders. Combined with data from human and nonhuman primate (not reviewed here), research in this extremely active field has implications both for our understanding of the biology of addiction and for therapeutic interventions to treat the disorder.

Progress in genetic studies of pain and analgesia

Interindividual variability in pain sensitivity and the response to analgesic manipulations remains a considerable clinical challenge as well as an area of intense scientific investigation. Techniques in this field have matured rapidly so that much relevant data have emerged only in the past few years. Our increasing understanding of the genetic mediation of these biological phenomena have nonetheless revealed their surprising complexity. This review provides a comprehensive picture and critical analysis of the field and its prospects.

Multivariate analyses reveal common and drug-specific genetic influences on responses to four drugs of abuse

Vulnerability to abused drugs is influenced by multiple genes unique to each drug and to risk genes for polydrug abuse. If several inbred mouse strains respond to different drugs similarly, this implies the action of a common group of genes. Simultaneous analysis of multiple responses to multiple drugs has been attempted infrequently. We performed multivariate analyses of published strain responses to four drugs. Genetic similarity in responses did not simply track pharmacological class. Withdrawal severity and preference for ethanol and diazepam were affected by many genes in common, although inversely. We focused on behavioral responses, but there is a growing archival database of physiological, pharmacological and biochemical strain traits. The genomics community is increasingly focusing on single-nucleotide polymorphism and haplotype-based gene mapping approaches, for which inbred strain data are also useful. Thus, similar analyses should be applicable to other laboratories, traits and genotypes.

Mu-opioid receptor activation induces transcriptional plasticity in the central extended amygdala

Addiction develops from the gradual adaptation of the brain to chronic drug exposure, and involves genetic reprogramming of neuronal function. The central extended amygdala (EAc) is a network formed by the central amygdala and the bed nucleus of the stria terminalis. This key site controls drug craving and seeking behaviors, and has not been investigated at the gene regulation level. We used Affymetrix microarrays to analyze transcriptional activity in the murine EAc, with a focus on mu-opioid receptor-associated events because these receptors mediate drug reward and dependence. We identified 132 genes whose expression is regulated by a chronic escalating morphine regimen in the EAc from wild-type but not mu-opioid receptor knockout mice. These modifications are mostly EAc-specific. Gene ontology analysis reveals an overrepresentation of neurogenesis, cell growth and signaling protein categories. A separate quantitative PCR analysis of genes in the last of these groups confirms the dysregulation of both orphan (Gpr88) and known (DrD1A, Adora2A, Cnr1, Grm5, Gpr6) G protein-coupled receptors, scaffolding (PSD95, Homer1) and signaling (Sgk, Cap1) proteins, and neuropeptides (CCK, galanin). These transcriptional modifications do not occur following a single morphine injection, and hence result from long-term adaptation to excessive mu receptor activation. Proteins encoded by these genes are classically associated with spine modules function in other brain areas, and therefore our data suggest a remodeling of EAc circuits at sites where glutamatergic and monoaminergic afferences interact. Together, mu receptor-dependent genes identified in this study potentially contribute to drug-induced neural plasticity, and provide a unique molecular repertoire towards understanding drug craving and relapse.

Cannabinoids in health and disease

Cannabis sativa L. preparations have been used in medicine for millenia. However, concern over the dangers of abuse led to the banning of the medicinal use of marijuana in most countries in the 1930s. Only recently, marijuana and individual natural and synthetic cannabinoid receptor agonists and antagonists, as well as chemically related compounds, whose mechanism of action is still obscure, have come back to being considered of therapeutic value. However, their use is highly restricted. Despite the mild addiction to cannabis and the possible enhancement of addiction to other substances of abuse, when combined with cannabis, the therapeutic value of cannabinoids is too high to be put aside. Numerous diseases, such as anorexia, emesis, pain, inflammation, multiple sclerosis, neurodegenerative disorders (Parkinson's disease, Huntington's disease, Tourette's syndrome, Alzheimer's disease), epilepsy, glaucoma, osteoporosis, schizophrenia, cardiovascular disorders, cancer, obesity, and metabolic syndrome-related disorders, to name just a few, are being treated or have the potential to be treated by cannabinoid agonists/antagonists/cannabinoid-related compounds. In view of the very low toxicity and the generally benign side effects of this group of compounds, neglecting or denying their clinical potential is unacceptable--instead, we need to work on the development of more selective cannabinoid receptor agonists/antagonists and related compounds, as well as on novel drugs of this family with better selectivity, distribution patterns, and pharmacokinetics, and--in cases where it is impossible to separate the desired clinical action and the psychoactivity--just to monitor these side effects carefully.

Homers regulate drug-induced neuroplasticity: implications for addiction

Drug addiction is a chronic, relapsing disorder, characterized by an uncontrollable motivation to seek and use drugs. Converging clinical and preclinical observations implicate pathologies within the corticolimbic glutamate system in the genetic predisposition to, and the development of, an addicted phenotype. Such observations pose cellular factors regulating glutamate transmission as likely molecular candidates in the etiology of addiction. Members of the Homer family of proteins regulate signal transduction through, and the trafficking of, glutamate receptors, as well as maintain and regulate extracellular glutamate levels in corticolimbic brain regions. This review summarizes the existing data implicating the Homer family of protein in acute behavioral and neurochemical sensitivity to drugs of abuse, the development of drug-induced neuroplasticity, as well as other behavioral and cognitive pathologies associated with an addicted state.

Transcriptional profiling of C57 and DBA strains of mice in the absence and presence of morphine

Background

The mouse C57BL/6 (C57) and DBA/2J (DBA) inbred strains differ substantially in many aspects of their response to drugs of abuse. The development of microarray analyses represents a genome-wide method for measuring differences across strains, focusing on expression differences. In the current study, we carried out microarray analysis in C57 and DBA mice in the nucleus accumbens of drug-naïve and morphine-treated animals.

Results

We identified mRNAs with altered expression between the two strains. We validated the mRNA expression changes of several such mRNAs, including Gnb1, which has been observed to be regulated by several drugs of abuse. In addition, we validated alterations in the enzyme activity of one mRNA product, catechol-O-methyltransferase (Comt). Data mining of expression and behavioral data indicates that both Gnb1 and Comt expression correlate with aspects of drug response in C57/DBA recombinant inbred strains. Pathway analysis was carried out to identify pathways showing significant alterations as a result of treatment and/or due to strain differences. These analyses identified axon guidance genes, particularly the semaphorins, as showing altered expression in the presence of morphine, and plasticity genes as showing altered expression across strains. Pathway analysis of genes showing strain by treatment interaction suggest that the phosphatidylinositol signaling pathway may represent an important difference between the strains as related to morphine exposure.

Conclusion

mRNAs with differing expression between the two strains could potentially contribute to strain-specific responses to drugs of abuse. One such mRNA is Comt and we hypothesize that altered expression of Comt may represent a potential mechanism for regulating the effect of, and response to, multiple substances of abuse. Similarly, a role for Gnb1 in responses to multiple drugs of abuse is supported by expression data from our study and from other studies. Finally, the data support a role for semaphorin signaling in morphine effects, and indicate that altered expression of genes involved in phosphatidylinositol signaling and plasticity might also affect the altered drug responses in the two strains.